This invention relates to phased array systems. With greater specificity, but without limitation thereto, the invention relates to phased array systems that employ the intrinsic synchronization properties of nonlinear oscillators. With further specificity, but without limitation thereto, the invention relates to using the intrinsic synchronization properties of nonlinear oscillators in phased array systems to provide simultaneous beam forming and beam shaping to such systems.
Traditionally, passive sensor or radiative arrays have employed linear, independently controlled transducers (also known as “radiators”) as the constituent elements of the array. The geometry of these elements controls the radiation of the beam pattern and signal processing gain. Classic phased array aperture (antenna) operation in a receiving mode can be broken into four steps: 1) transduce the received energy; 2) synchronously demodulate the transduced signal; 3) apply weights to phase shift the inputs from each of the transducer elements; and 4) sum the weighted signals together to produce an output signal. The maximum gain possible is proportional to the number of antenna elements. Reciprocity permits the process to be reversed for the transmission of signals.
Implicit in the traditional phased array aperture (antenna) design is that each transducer is either assumed or engineered to be linear and to operate independently of (without an input from) the other transducers in the array. Due to these assumptions, interactions (i.e., “mutual coupling”) between array elements are viewed negatively, as such interactions frustrate the formation of a desired antenna pattern. Mitigation of these mutual radiative coupling effects typically requires that transducer spacing be limited to a minimum of half a wavelength of the lowest frequency the array is designed to receive or transmit.
In such arrays, electronic beam steering (or beam scanning) is commonly realized through use of a phase-shifter at each transducer element. A computer typically controls each phase shifter, with control lines to each element being used to program the phase of each individual element. Unfortunately, the phase-shifters add to the weight, power losses, operating power, size, complexity and, significantly, to the cost of the phased array. For certain applications, one or more of these factors will eliminate the viability of using phase-shifters for beam steering an array.
To this end, solutions to phase-shifterless beam steering have been investigated. Several alternatives exist, such as frequency-scanning and multiple beam-forming networks (e.g. Rotman lenses and Butler matrices).
One of the earliest attempts to exploit synchronization for phase-shifterless beam steering was made by Stephan and Morgan (see K. D. Stephan and W. A. Morgan, “Analysis of Inter-Injection-Locked Oscillators for Integrated Phased Arrays”, IEEE Transactions of Antennas and Propagation, vol. AP-35, pp. 771–781, July 1987). By injecting a sinusoidal signal into the two elements at each end of a 1-dimensional array of intentionally coupled oscillators, Stephan et al showed that the phase distribution across the array will evolve toward a uniform phase gradient. Although experimentally successful, the scan angles are inversely dependent on the number of elements in this interinjection-locking technique. Consequently, the scanning ability of large arrays is very limited.
Professor Robert York of the University of California, Santa Barbara, has suggested an alternative approach. Professor York and his co-workers also utilized an array of nonlinear oscillators. These oscillators were interactively coupled by what is known in the art as “nearest neighbor” coupling. This alternative method does not rely on signal injection (see P. Liao and R. A. York, “A new phase-shifterless beam-scanning technique using arrays of coupled oscillators”, IEEE Transactions of Microwave Theory and Techniques, vol. 41, pp. 1810–1815, October 1993). Instead, York and company demonstrated both experimentally and analytically that oscillator phase distribution could be manipulated through control of the oscillator natural frequencies. Unlike Stephan's method, York's approach yields phase gradient values between ±90 regardless of the number of oscillators used.
While advances have been made in phase-shifterless beam steering approaches, there is a continued desire to expand the capabilities of such approaches. Besides providing a phase shifterless array system having enhanced scanning capabilities, there is also a desire to provide beam shaping to the attendant beam that is steered, so that simultaneous beam steering and beam shaping (i.e. sidelobe reduction) is possible.